1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell stack, a heat exchanger, and a reformer provided in a casing.
2. Description of the Related Art
Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined number of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. At the anode, oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
For example, a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 5-47409 is known. In the conventional fuel cell, as shown in FIG. 17, a plurality of cell stacks 2 are disposed in a casing 1. The cell stacks 2 are tightened together by tightening bolts 3 and belleville springs 4.
A fuel gas supply pipe 5a, an oxygen-containing gas supply pipe 6a, and an oxygen-containing gas discharge pipe 6b are connected to the cell stacks 2. The fuel gas supply pipe 5a, the oxygen-containing gas supply pipe 6a, and the oxygen-containing gas discharge pipe 6b extend though the casing 1 to the outside. A fuel gas discharge pipe 5b is attached to the casing 1. The fuel gas discharge pipe 5b is opened to the inside of the casing 1.
Each of the cell stacks 2 is formed by stacking a plurality of unit cells 7 vertically in a stacking direction. At opposite ends of the cell stack 2 in the stacking direction, end plates 8a, 8b are provided. The fuel gas supply pipe 5a, the oxygen-containing gas supply pipe 6a, and the oxygen-containing gas discharge pipe 6b are connected to the end plate 8a. The end plate 8b is placed on the bottom surface of the casing 1.
In the conventional technique, the fuel gas and the oxygen-containing gas are supplied from the fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a into the respective cell stacks 2 through the end plates 8a. At this time, since the temperatures of the fuel gas and the oxygen-containing gas are lower than the temperature of the cell stacks 2, the temperature at positions near the reactant gas inlets of the end plates 8a decreases undesirably.
The fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a are connected to each of the end plates 8a. Heat radiation from the end plates 8a occurs easily through the fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a. Thus, the heat efficiency is poor. Further, in the area where the end plate 8a is connected to the fuel gas supply pipe 5a and the oxygen-containing gas supply pipe 6a, the temperature is low in comparison with the other area. Therefore, the temperature varies significantly depending on the area of the end plate 8a, and distortion may occur in the end plate 8a undesirably.
Further, dedicated space is needed for each of the fuel gas supply pipe 5a, the oxygen-containing gas supply pipe 6a, and the oxygen-containing gas discharge pipe 6b in the casing 1. Therefore, the size of the casing 1 is considerably large. The surface area of the casing 1 is large, and the heat efficiency is poor.